Solid-state Li/LiFePO4 polymer electrolyte batteries incorporating an ionic liquid cycled at 40 °C

The cycle behaviour and rate performance of solid-state Li/LiFePO₄ polymer electrolyte batteries incorporating the N-methyl-N-propylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR₁₃TFSI) room temperature ionic liquid (IL) into the P(EO)₂₀LiTFSI electrolyte and the cathode have been investigated at 40 °C. The ionic conductivity of the P(EO)₂₀LiTFSI + PYR₁₃TFSI polymer electrolyte was about 6 × 10⁻⁴ S cm⁻¹ at 40 °C for a PYR₁₃⁺/Li⁺ mole ratio of 1.73. Li/LiFePO₄ batteries retained about 86% of their initial discharge capacity (127 mAh g⁻¹) after 240 continuous cycles and showed excellent reversible cyclability with a capacity fade lower than 0.06% per cycle over about 500 cycles at various current densities. In addition, the Li/LiFePO₄ batteries exhibited some discharge capability at high currents up to 1.52 mA cm⁻² (2 C) at 40 °C which is very significant for a lithium metal-polymer electrolyte (solvent-free) battery systems. The addition of the IL to lithium metal-polymer electrolyte batteries has resulted in a very promising improvement in performance at moderate temperatures.     

Ionic liquid and plastic crystalline phases of pyrazolium imide salts as electrolytes for rechargeable lithium-ion batteries

A new member of the plastic crystal, pyrazolium imide family, N,N′-diethyl-3-methylpyrazolium bis-(trifluoromethanesulfonyl)imide (DEMPyr123) was prepared. It showed a single, plastic crystalline phase that extends from 4.2 °C to its melting at 11.3 °C. When 10 mol% LiTFSI salt was added, the mixture showed ionic conductivities reaching 1.7 × 10⁻³ S cm⁻¹ at 20 °C, in the liquid state and 6.9 × 10⁻⁴ S cm⁻¹ at 5 °C, in the solid, plastic phase. A wide electrochemical stability window's of 5.5 V was observed by cyclic voltammetry of the molten salt mixture. Batteries were assembled with LiFePO₄/Li₄Ti₅O₁₂ electrodes and the salt mixture as an electrolyte. They showed a charge/discharge efficiency of 93% and 87% in the liquid and the plastic phase, respectively. The capacity retention was very good in both states with 90% of the initial capacity still available after 40 cycles. In general, the batteries showed good rate capability and cycle life performance in the ionic liquid phase that were sustained when the electrolyte transformed to the plastic phase. Comparison of the battery results with those of a classic (non-plastic crystal) ionic liquid has proven the advantage of the dual state of matter character in this electrolyte.     

Lithium ion and electronic conductivity in 3-(oligoethylene oxide)thiophene comb-like polymers

The Li-ion and electronic conductivities of a series of p-doped poly(thiophene)s with oligo-ethylene oxide side chains have been determined at room temperature as functions of side-chain length and concentration of LiOTf dissolved in the polymers in order to assess their utility as binders in Li-ion batteries. The lithium triflate concentration was varied from 0.23 to 2.26 mmol LiOTf/g –C₂H₄O– (100 O:Li to 10 O:Li), and the concentration of dissociated Li⁺ was determined from the IR spectra of the polymer solutions. The greatest ionic conductivity, 2 × 10⁻⁴ S cm⁻¹, was attained with intermediate concentrations of added salt that corresponded with the greatest degree of LiOTf dissociation. Li-ion mobilities of 5 × 10⁻⁷ cm² (Vs)⁻¹ were measured for poly(thiophene)s (PT) with short oligo(ethylene oxide) side-chains (E_n), PE₂T and PE₃T, whereas the polymers with longer side chains, PE₇T and PE₁₅T, had Li-ion mobilities about an order of magnitude greater, 5 × 10⁻⁶ cm² (Vs)⁻¹. The electronic conductivity of the polymers heavily doped with NOBF₄ was near 0.1 S cm⁻¹ for PE₂T and PE₃T, but was orders of magnitude smaller for the polymers with longer side-chains. Addition of LiOTf caused the electronic conductivity of PE₂T and PE₃T to drop to that of the longer chain polymers whose conductivities were insensitive to the LiOTf concentration.     

Cracking causing cyclic instability of LiFePO4 cathode material

The capacity of pure LiFePO₄ faded gradually from initial 149 mAh g⁻¹–117 mAh g⁻¹ under current density of 30 mA g⁻¹ at room temperature after 60 cycles. Some obvious cracks are observed in LiFePO₄ particles after cycling. The formation of cracks would lead to poor electric contact and capacity fading. A possible mechanism is proposed for the appearance of the cracks.     

Effects of ball-milling on lithium insertion into multi-walled carbon nanotubes synthesized by thermal chemical vapour deposition

The effects of ball-milling on Li insertion into multi-walled carbon nanotubes (MWNTs) are presented. The MWNTs are synthesized on supported catalysts by thermal chemical vapour deposition, purified, and mechanically ball-milled by the high energy ball-milling. The purified MWNTs and the ball-milled MWNTs were electrochemically inserted with Li. Structural and chemical modifications in the ball-milled MWNTs change the insertion–extraction properties of Li ions into/from the ball-milled MWNTs. The reversible capacity (C_rev) increases with increasing ball-milling time, namely, from 351 mAh g⁻¹ (Li_0.9C₆) for the purified MWNTs to 641 mAh g⁻¹ (Li_1.7C₆) for the ball-milled MWNTs. The undesirable irreversible capacity (C_irr) decreases continuously with increase in the ball-milling time, namely, from 1012 mAh g⁻¹ (Li_2.7C₆) for the purified MWNTs to 518 mAh g⁻¹ (Li_1.4C₆) for the ball-milled MWNTs. The decrease in C_irr of the ball-milled samples results in an increase in the coulombic efficiency from 25% for the purified samples to 50% for the ball-milled samples. In addition, the ball-milled samples maintain a more stable capacity than the purified samples during charge–discharge cycling.     

Preparation and characterization of high-density spherical Li4Ti5O12 anode material for lithium secondary batteries

Li₄Ti₅O₁₂ is a very promising anode material for lithium secondary batteries. A novel technique has been developed to prepare Li₄Ti₅O₁₂. The spherical precursor is prepared via an “inner gel” method by TiCl₄ as the raw material. Spherical Li₄Ti₅O₁₂ powders are synthesized by sintering the mixture of spherical precursor and Li₂CO₃. The investigation of XRD, SEM and the determination of the electrochemical properties show that the Li₄Ti₅O₁₂ powders prepared by this method are spherical, and have high tap-density and excellent electrochemical performance. It is tested that the tap-density of the product is as high as 1.64 g cm⁻³, which is remarkably higher than the non spherical Li₄Ti₅O₁₂. Between 1.0 and 3.0 V versus Li, a reversible capacity is as high as 161 mAh g⁻¹ at a current density of 0.08 mA cm⁻².     

Advanced manganese oxide material for rechargeable lithium cells

A family of potassium-doped manganese oxide materials were synthesized with the stoichiometric formula Li_0.9−XK_XMn₂O₄, where X = 0.0–0.25 and evaluated for their viability as a cathode material for a rechargeable lithium battery. A performance maximum was found at X = 0.1 where the initial specific capacity for the lithium–potassium-doped manganese dioxide electrochemical couple was 130 mAh g⁻¹ of active cathode material. The discharge capacity of the system was maintained through 90 cycles (95% initial capacity). Additionally, the capacity was maintained at greater than 90% initial discharge through 200 cycles. Other variants demonstrated greater than 75% initial discharge through 200 cycles at comparable capacity.     

Li+ ion diffusion in LiMn2O4 thin film prepared by PVP sol–gel method

LiMn₂O₄ thin film (1 μm thick) was prepared on a gold substrate by the PVP sol–gel method. The electrochemical properties of the thin-film electrode were studied in an electrolyte 1 mol dm⁻³ LiClO₄/(ethylene carbonate + diethyl carbonate). The prepared LiMn₂O₄ showed a good charge–discharge performance, and the capacity fade was ca. 20% during 200 cycles. The Li⁺ ion diffusion in the LiMn₂O₄ thin film was investigated by means of potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. The chemical diffusion coefficients were estimated to be 10⁻⁸ to 10⁻¹⁰ cm² s⁻¹.     

Effect of organic acids and nano-sized ceramic doping on PEO-based solid polymer electrolytes

Composite solid polymer electrolytes (CSPEs) consisting of polyethyleneoxide (PEO), LiClO₄, organic acids (malonic, maleic, and carboxylic acids), and/or Al₂O₃ were prepared in acetonitrile. CSPEs were characterized by Brewster Angle Microscopy (BAM), thermal analysis, ac impedance, cyclic voltammetry, and tested for charge–discharge capacity with the Li or LiNi_0.5Co_0.5O₂ electrodes coated on stainless steel (SS). The morphologies of the CSPE films were homogeneous and porous. The differential scanning calorimetric (DSC) results suggested that performance of the CSPE film was highly enhanced by the acid and inorganic additives. The composite membrane doped with organic acids and ceramic showed good conductivity and thermal stability. The ac impedance data, processed by non-linear least square (NLLS) fitting, showed good conducting properties of the composite films. The ionic conductivity of the film consisting of (PEO)₈LiClO₄:citric acid (99.95:0.05, w/w%) was 3.25 × 10⁻⁴ S cm⁻¹ and 1.81 × 10⁻⁴ S cm⁻¹ at 30 °C. The conductivity has further improved to 3.81 × 10⁻⁴ S cm⁻¹ at 20 °C by adding 20 w/w% Al₂O₃ filler to the (PEO)₈LiClO₄ + 0.05% carboxylic acid composite. The experimental data for the full cell showed an upper limit voltage window of 4.7 V versus Li/Li⁺ for CSPE at room temperature.     

Electrochemical properties of iodine-containing lithium manganese oxide spinel

Iodine-containing, cation-deficient, lithium manganese oxides (ICCD-LMO) are prepared by reaction of MnO₂ with LiI. The MnO₂ is completely transformed into spinel-structured compounds with a nominal composition of Li_1−δMn_2−2δO₄I_x. A sample prepared at 800 °C, viz. Li_0.99Mn_1.98O₄I_0.02, exhibits an initial discharge capacity of 113 mA h g⁻¹ with good cycleability and rate capability in the 4-V region. Iodine-containing, lithium-rich lithium manganese oxides (ICLR-LMO) are also prepared by reaction of LiMn₂O₄ with LiI, which results in a nominal composition of Li_1+xMn_2−xO₄I_x. Li_1.01Mn_1.99O₄I_0.02 shows a discharge capacity of 124 mA h g⁻¹ on the first cycle and 119 mA h g⁻¹ a on the 20th cycle. Both results indicate that a small amount of iodine species helps to maintain cycle performance.     

Structural and ionic transport properties of Li2AlZr[PO4]3

The fast ionic conductor Li₂AlZr[PO₄]₃ has been prepared by the solid state reaction method. The formation of the compound is confirmed by XRD and FTIR analysis. The system has been subjected to ac conductivity measurements in the temperature range 523 to 623 K with aluminium as blocking electrodes over a frequency range of 42 Hz to 5 MHz. The conductivity is found to be 1 × 10⁻⁵ S cm⁻¹ at 623 K. The activation energy calculated from the Arrhenius plot is 0.83 eV. The conductance spectrum reveals a d.c. plateau and a dispersive region that suggest the correlated hopping motion of ions. Thus, the conduction mechanism in Li₂AlZr[PO₄]₃ may be due to the hopping of charge carriers.     

Opportunity of metallic interconnects for ITSOFC: Reactivity and electrical property

Iron-base alloys (Fe–Cr) are proposed hereafter as materials for interconnect of planar-type intermediate temperature solid oxide fuel cell (ITSOFC); they are an alternative solution instead of the use of ceramic interconnects. These steels form an oxide layer (chromia) which protects the interconnect from the exterior environment, but is an electrical insulator. One solution envisaged in this work is the deposition of a reactive element oxide coating, that slows down the formation of the oxide layer and that increases its electric conductivity. The oxide layer, formed at high temperature on the uncoated alloys, is mainly composed of chromia; it grows in accordance with the parabolic rate law (k_p = 1.4 × 10⁻¹² g² cm⁻⁴ s⁻¹). On the reactive element oxide-coated alloy, the parabolic rate constant, k_p, decreases to 1.3 × 10⁻¹³ g² cm⁻⁴ s⁻¹. At 800 °C, the area-specific resistance of Fe–30Cr alloys is about 0.03 Ω cm² after 24 h in laboratory air under atmospheric pressure. The Y₂O₃ coating reduces the electrical resistance 10-fold. This indicates that the application of Y₂O₃ coatings on Fe–30Cr alloy allows to use it as an interconnect for SOFC.     

Preparation and properties of ceramic interconnecting materials,La0.7Ca0.3CrO3−δ doped with GDC for IT-SOFCs

One of the challenges for improving the performance and cost-effectiveness of solid oxide fuel cells (SOFCs) is the development of effective interconnect materials. A widely used interconnect ceramic for SOFCs is doped lanthanum chromite. In this paper, we report a doped lanthanum chromite, La_0.7Ca_0.3CrO_3−δ (LCC) + x wt.% Gd_0.2Ce_0.8O_1.9 (GDC) (x = 0–10), with improved electrical conductivity and sintering capability. In this composite material system, LCC + GDC were prepared by an auto-ignition process and the electrical conductivity was characterized in air and in H₂. The LCC powders exhibited a better sintering ability and could reach a 94.7% relative density at 1400 °C for 4 h in air and with the increase of GDC content the relative density increased, reached 98.5% when the GDC content was up to 10 wt.%. The electrical conductivity of the samples dramatically increased with GDC addition until a maximum of 134.48 S cm⁻¹ in air at 900 °C when the materials contained 3 wt.% GDC. This is 5.5 times higher than pure LCC (24.63 S cm⁻¹). For the sample with a 1 wt.% GDC content, the conductivity in pure H₂ at 900 °C was a maximum 5.45 S cm⁻¹, which is also higher than that of pure LCC ceramics (4.72 S cm⁻¹). The average thermal expansion coefficient (TEC) increased with the increase of GDC content, ranging from 11.12 to 14.32 × 10⁻⁶ K⁻¹, the majority of which unfortunately did not match that of 8YSZ. The oxygen permeation measurement presented a negligible oxygen ionic conduction, indicating that it is still an electronically conducting ceramic. Therefore, it is a very promising interconnect material for higher performance and cost-effectiveness for SOFCs.     

Effect of Fe2P on the electron conductivity and electrochemical performance of LiFePO4 synthesized by mechanical alloying using Fe3+ raw material

LiFePO₄ and LiFePO₄/Fe₂P composites have been produced using raw Fe₂O₃ materials by mechanical alloying (MA) and subsequent firing at 900 °C. The LiFePO₄ prepared by firing at 900 °C for 30 min showed a maximum discharge capacity of 160 mAh g⁻¹ at C/20, which is at a higher capacity and improved cell performance compared with the LiFePO₄ prepared using for a longer firing times. LiFePO₄/Fe₂P composites have been synthesized by the reduction reaction of phosphate in excess of carbon. By transmission electron microscopy (TEM) and scanning electron microscopy (SEM) it was determined that the LiFePO₄ phase was agglomerated with a primary particle size of 40–50 nm around the surface of Fe₂P with particle size of 200 nm. The electronic conductivity of the LiFePO₄/Fe₂P composite increased in proportion with the amount that the Fe₂P phase and discharge capacity increased during the cycling. The sample containing 8% of Fe₂P in LiFePO₄/Fe₂P composite showed a high discharge capacity and rate capability at high current.     

Metal oxyfluorides TiOF2 and NbO2F as anodes for Li-ion batteries

Lithium insertion and extraction in to/from the oxyfluorides TiOF₂ and NbO₂F is investigated by galvanostatic cycling, cyclic voltammetry and impedance spectroscopy in cells using Li-metal as a counter electrode at ambient temperature. The host compounds are prepared by low-temperature reaction and characterized by powder X-ray diffraction (XRD), Rietveld refinement and Brunauer, Emmett and Teller (BET) surface area. Crystal structure destruction occurs during the first-discharge reaction with Li at voltages below 0.8–0.9 V for Li_xTiOF₂ as shown by ex situ XRD and at ≤1.4 V for Li_xNbO₂F to form amorphous composites, ‘Li_xTi/NbO_y–LiF’. Galvanostatic discharge–charge cycling of ‘Li_xTiO_y’ in the range 0.005–3.0 V at a current density of 65 mA g⁻¹ gives a capacity of 400 (±5) mAh g⁻¹ during 5–100 cycles with no noticeable capacity fading. This value corresponds to 1.52 mol of recycleable Li/Ti. The coulombic efficiency (η) is >98%. Results on ‘Li_xNbO_y’ show good reversibility of the electrode and a η >98% is achieved only after 10 cycles (range 0.005–3.0 V and at 30 mA g⁻¹) and a capacity of 180 (±5) mAh g⁻¹ (0.97 mol of Li/Nb) was stable up to 40 cycles. In both ‘Li_xTiO_y’ and ‘Li_xNbO_y’, the average discharge and charge voltages are 1.2–1.4 and 1.7–1.8 V, respectively. The impedance spectral data measured during the first cycle and after selected numbers of cycles are fitted to an equivalent circuit and the roles played by the relevant parameters as a function of cycle number are discussed.     

Porous olivine composites synthesized by sol–gel technique

Porous LiMPO₄/C composites (where M stands for Fe and/or Mn) with micro-sized particles were synthesised by sol–gel technique. Particles porosity is discussed in terms of qualitative results obtained from SEM micrographs and in terms of quantitative results obtained from N₂ adsorption isotherms. Porous particles could be described as an inverse picture of nanoparticulate arrangement, where the pores serve as channels for lithium supply and the distance between the pores determines the materials kinetics. Tests show that the electrochemical behaviour of porous LiMPO₄/C composite is comparable with the results from the literature. The best electrochemical results were obtained with a LiFePO₄/C composite (over 140 mAh g⁻¹ at C/2 rate during continuous cycling). The capacity obtained with LiMnPO₄/C composite is much lower (40 mAh g⁻¹ at C/20 rate), although the textural properties are similar to those observed in the LiFePO₄/C composite.     

Synthesis and characterization of spinel Li4Ti5O12 anode material by oxalic acid-assisted sol–gel method

Anode material Li₄Ti₅O₁₂ for lithium-ion batteries has been prepared by a novel sol–gel method with oxalic acid as a chelating agent and Li₂CO₃ and tetrabutyl titanate [Ti(OC₄H₉)₄] as starting materials. Various initial conditions were studied in order to find the optimal conditions for the synthesis of Li₄Ti₅O₁₂. Oxalic acid used in this method functioned as a fuel, decomposed the metal complexes at low temperature and yielded the free impurity Li₄Ti₅O₁₂ compounds. Thermal analyses (TG–DTA) and XRD data show that powders grown with a spinel structure (Fd3m space group) have been obtained at 800 °C for 16 h. SEM analyses indicated that the prepared Li₄Ti₅O₁₂ powders had a uniform cubic morphology with average particle size of 200 nm. The influence of synthesis conditions on the electrochemical properties was investigated and discussed. The discharge capacity of Li₄Ti₅O₁₂ synthesized with an oxalic acid to titanium ratio R = 1.0 was 171 mAh g⁻¹ in the first cycle and 150 mAh g⁻¹ after 35 cycles under an optimal synthesis condition at 800 °C for 20 h. The very flat discharge and charge curves indicated that the electrochemical reaction based on Ti⁴⁺/Ti³⁺redox couple was a typical two-phase reaction.     

Ionic conductance of PDMAEMA/PEO polymeric electrolyte containing lithium salt mixed with plasticizer

Novel plasticized polymer electrolytes were synthesized with poly(N,N-dimethylamino-ethyl-methacrylate) (PDMAEMA), polyethylene oxide (PEO), LiTFSI as a salt, tetraethylene glycol dimethyl ether (tetraglyme), EC/PC and DEP as plasticizers. The ionic conductivity of various compositions of polymer electrolytes was investigated as a function of temperature, various concentrations of LiTFSI, plasticizers and various ratio of PDMAEMA/PEO. The ionic conductivity of PDMAEMA/PEO/LiTFSI (1.5 mol kg⁻¹) with DEP as a plasticizer (1.5 × 10⁻⁴ S cm⁻¹) exhibited lower than PDMAEMA/PEO/LiTFSI (1.2 mol kg⁻¹)/tetraglyme (5.24 × 10⁻⁴ S cm⁻¹) and PDMAEMA/PEO/LiTFSI (1.5 mol kg⁻¹)/EC + PC (2.1 × 10⁻⁴ S cm⁻¹). As increasing the PDMAEMA concentration up to 13.3%, the ionic conductivity was decreased rapidly. As increasing the PDMAEMA concentration the ionic conductivity was decreased due to high viscosity and some interactions reducing ion pairing. These plasticized polymer electrolytes were characterized by impedance spectroscopy and DSC.     

Preparation and electrochemical studies of Fe-doped Li3V2(PO4)3 cathode materials for lithium-ion batteries

The Fe-doped Li₃V₂(PO₄)₃ cathode materials for Li-ion batteries were synthesized by a conventional solid-state reaction, and the Fe-doping effects on the Li electrochemical extraction/insertion performance of Li₃V₂(PO₄)₃ were investigated by galvanostatic charge/discharge and cyclic voltammetry measurements. The optimal Fe-doping content x is 0.02–0.04 in Li₃Fe_xV_2−x(PO₄)₃ system. The Fe-doped Li₃V₂(PO₄)₃ samples showed a better cyclic ability between 3.0 and 4.9 V, for example, the discharge capacity of Li₃Fe_0.02V_1.98(PO₄)₃ was 177 mAh g⁻¹ in the 1st cycle and 126 mAh g⁻¹ in the 80th cycle. The retention rate of discharge capacity is about 71%, much higher than 58% of the undoped system. The improved electrochemical performances of the Li₃V₂(PO₄)₃ could be attributed to the increased electrical conductivity and structural stability deriving from the incorporation of the Fe³⁺ ions.